CD22 is a 135-kD type I transmembrane sialoglycoprotein of the immunoglobulin (Ig) superfamily. CD22 expression is specific to B cells and is developmentally regulated so that expression is limited in pro-B and pre-B cells (Dorner & Goldenberg, 2007, Ther Clin Risk Manag 3:954-59). As B cells mature, expression increases and localization of CD22 shifts to the cell surface (Dorner & Goldenberg, 2007). CD22 is strongly expressed on follicular, mantle and marginal-zone B cells, but is weakly present in germinal B cells (Dorner & Goldenberg, 2007). CD22 is an inhibitory co-receptor that downmodulates B-cell receptor (BCR) signaling by setting a signaling threshold that prevents overstimulation of B cells (Nitschke, 2005, Curr Opin Immunol 17:290-97).
Antibodies against CD22, such as epratuzumab (hLL2), have been used for treatment of a variety of cancers and autoimmune diseases, including but not limited to acute lymphoblastic leukemia (Hoelzer et al., 2013, Curr Opin Oncol 25:701-6), chronic lymphocytic leukemia (Macromatis & Cheson, 2004, Blood Rev 18:137-48), non-Hodgkin's lymphoma (Leonard et al., 2004, Clin Cancer Res 10:5327-34; Dorner & Goldenberg, 2007), follicular lymphoma (Illidge & Morchhauser, 2011, Best Pract Res Clin Haematol 24:279-93), diffuse large B-cell lymphoma (Micallef et al., 2011, Blood 118:4053-61), mantle cell lymphoma (Sharkey et al., 2012, Mol Cancer Ther 11:224-34), systemic lupus erythematosus (Dorner & Goldenberg, 2007; Strand et al., 2013, Rheumatology Nov. 22, 2013 Epub ahead of print; Wallace & Goldenberg, 2013, Lupus 22:400-5; Wallace et al., 2013, Rheumatology 52:1313-22; Wallace et al., 2014, Ann Rheum Dis 73:183-90), and primary Sjögren's syndrome (Steinfeld et al., 2006, Arthritis Res Ther 8:R129; Dorner & Goldenberg, 2007). A phase III clinical trial of epratuzumab in systemic lupus erythematosus is currently in progress (see, e.g., ClinicalTrials.gov, “Study of Epratuzumab versus Placebo in Subjects with Moderate to Severe General Systemic Lupus Erythematosus (EMBODY 1)”). Because CD22 regulates B-cell functions and survival, it is an important link for modulating humoral immunity and proliferation of B-cell lymphomas and a target for therapeutic antibodies in cancer and autoimmune disease (Dorner & Goldenberg, 2007).
RFB4 is a murine anti-CD22 antibody that was initially thought to bind to the same epitope group (epitope B) of CD22 as epratuzumab (Stein et al., 1993, Cancer Immunol Immunother 37:293-8). However, more recent studies provided evidence that RFB4 and epratuzumab respectively bind to domain 3 and domain 2 of CD22. A need exists for a humanized form of RFB4, with reduced immunogenicity in humans that shows enhanced B cell binding and efficacy against B cell related diseases, compared with epratuzumab.